The acrosome of a sperm must undergo exocytosis before the sperm fertilizes an egg. Freshly ejaculated mammalian sperm are not capable of exocytosis. They must first complete a poorly understood maturation process before they become acrosomally responsive. The long-term objective of this project is to understand the mechanisms by which exocytosis is controlled. Seminal plasma prevents sperm from becoming acrosomally responsive. The agents in seminal plasma that affect acrosomal responsiveness will be identified. This project will use seminal plasma and molecules isolated from it as tools to analyze the control of acrosomal responsiveness. Depending on how it is applied to sperm, seminal plasma can inhibit the development of acrosomal responsiveness, reverse acrosomal responsiveness, or augment acrosomal responsiveness. Because acrosomal exocytosis is triggered by an increased concentration of intracellular free Ca2+ and may require a rise in intracellular pH, the effect of seminal plasma on the intracellular concentrations of Ca2+ and H+ will be determined. The effect of SP on the fusibility of purified acrosomal and plasma membranes will also be tested. The rate at which human sperm become acrosomally responsive varies among men. Sperm of some men do not become responsive at a normal rate. This behavior is correlated with male subfertility, suggesting that inappropriate acrosomal exocytosis may contribute to male subfertility. A full understanding of the mechanisms by which acrosomal exocytosis is controlled will provide new insights into this class of human subfertility.